I view the Primo as an evolution not a revolution with many smaller changes. The cost difference between building the older Burly vs. the Primo is minimal. As for things addressed, the Primo does have an adjustable belt tightening system and mounting holes for end stops are included. The primo is generally stiffer, there is more ability to make adjustments during the build process, and it uses wider belts.
Generally parts from the Burly and Primo don’t mix. And even when they can mix, you lose some of the benefits. Mounts are a semi-exception. Ryan kept the mounting holes on the tubing the same, so it is possible to use the Burly mounting plate and use any Burly mount. Note that Ryan changed from using a Burly-style mounting plate to reduce the distance between the mount and the core. This means that the router is less cantilevered, so it is better to use a Primo-style mount if you can.
If you are starting from scratch, I’d build the Primo. There is going to be more support on this forum for the Primo going forward, and I cannot find a compelling argument for building the Burly instead.